The present invention relates to a backlash correcting device for a swivel arm of a robot.
The invention will be described with reference to an arc welding robot, especially, an articulated arc welding robot.
The swivel section of an articulated arc welding robot of a type extensively employed in the art is constructed as shown in FIG. 1. In the swivel section, in order to simplify the configuration and to minimize backlash, a special speed reducer, namely, a harmonic drive component, is employed, and a swivel shaft driving DC servo motor is provided on the swivel shaft. More specifically, as shown in FIG. 1, the swivel section 9, borne by bearings 10, is rotated through a speed reducer 2 by a motor 1. In this arrangement, it is possible for backlash to occur in the speed reducer 2, although the occurrence of backlash has been substantially eliminated because the manufacturing accuracy of speed reducers has been remarkably improved of late.
On the other hand, in the swivel section incorporating the swivel shaft driving DC motor, if the motor becomes out of order, it cannot be readily replaced, and accordingly the production line in which the robot is used must be stopped for a long period of time. In addition, it takes a significant amount of time to replace the brushes of the motor.
In order to eliminate this difficulty, recently robots have been provided with an external swivel-shaft driving DC servo motor mounted such that the motor can be readily replaced. An example of this type of arrangement in which the motor is mounted externally of the robot is shown in FIG. 2. In this arrangement, the output of an external motor 1 is transmitted through a speed reducer 2 to a gear shaft 5 engaged with a center gear 7 so that the latter rotates a swivel section 9 which is made rotatable with bearings 10. In this operation, backlash may occur mainly in the speed reducer 2 and in the gear shaft 5 and the center gear 7 at the point of engagement. The backlash of the speed reducer 2 has been substantially eliminated as was described above. However, significant backlash of the gear shaft and the center gear at the point of engagement can still occur if the gears are worn. As shown in FIG. 3A, two gears are correctly engaged with each other before they are worn. When the gears wear, a gap A (FIG. 3B) forms between the gears, as a result of which backlash occurs. Thus, it is necessary for a robot which has an external swivel shaft driving servo motor to provide for correction of backlash occurring between the motor shaft and the swivel output.